What context does save_as need?
Have you tried looking in the Blender documentation? It covers the following

bpy.ops.image.save_as(save_as_render=False, copy=False, filepath="", check_existing=True, filter_blender=False, filter_backup=False, filter_image=True, filter_movie=True, filter_python=False, filter_font=False, filter_sound=False, filter_text=False, filter_btx=False, filter_collada=False, filter_alembic=False, filter_folder=True, filter_blenlib=False, filemode=9, relative_path=True, show_multiview=False, use_multiview=False, display_type='DEFAULT', sort_method='FILE_SORT_ALPHA')